This invention relates to a rotation transmission device for transmitting driving force to the rear wheels of a four-wheel drive vehicle. More specifically, this device is used to selectively transmit the driving force when accelerating the vehicle and to cut off the driving force when braking.
A conventional four-wheel drive vehicle (hereinafter referred to as 4-WD), developed to increase the driving force when starting or accelerating and to improve the ability to keep moving, has a limited slip differential device such as a center differential or a viscous coupling between the front and rear axles. Such a limited slip differential device is not compatible with an antilock brake system (hereinafter referred to as ABS) for improving the braking force and driving stability, because such a combination tends to worsen the controllability by ABS and thus tends to cause simultaneous lockup of all four wheels or produce vibration of the vehicle body.
One conventional solution to this problem is shown in FIG. 25, in which on a rear wheel propeller shaft D of a 4-WD car, a one-way clutch G is provided between a viscous coupling F and a rear differential E to cut off the transmission of rotation while braking.
This arrangement utilizes the fact that, in an ordinary car, the front wheels tend to lock up before do the rear wheels when sharp brakes are applied. While the vehicle is moving forward at high speed, if sharp brakes are applied, the front wheels tend to lock up. But the one-way clutch G will begin idling instantly, thus cutting off the rotation restricting force between the front and rear wheels. This makes it possible for the ABS to detect the wheel speed difference between the front and rear wheels and perform antilock control.
But this arrangement has a problem in that when the vehicle is moving backward, that is, while the rear wheel propeller shaft D is rotating in reverse direction, the rear wheels cannot be driven because the one-way clutch G idles. Thus, no four-wheel drive is possible while the vehicle is moving backward.
If the vehicle is braked sharply while moving backward and the front wheels get locked, the one-way clutch G will engage, so that the rotation restricting force is produced by the differential limiting function of the viscous coupling F. This prevents the front and rear wheels from rotating at different speeds from each other. Thus, the ABS will lose its function because it is operative only if there is a rotational speed difference between wheels. Thus, there is a higher possibility of all four wheels getting locked simultaneously.